The invention relates to a headrest for a motor vehicle having the features of the preamble of claim 1.
A such type headrest is known from DE 102 15 054 A1. The first structural component is intended to be secured to a seat back of a motor vehicle seat, said first structural component including at least one bar. The second structural component comprises a bolster and if necessary further structural elements. In the normal state, the second structural component and as a result thereof the bolster as well, are in a normal position. In the event of a crash, the second structural component moves forward and, as a result thereof, gets in closer proximity to a passenger's head. This arrangement prevents what is termed a neck sprain/strain or whip lash in rear impact collisions. In the crash state, the second structural component is displaced some centimeters to the front and as far as practicable toward the top as well with respect to the normal state.
The drive unit is disposed between the first and the second structural component. It comprises a drive, for example a pressure storing means such as a spring. Said drive biases the second structural component with respect to the first structural component, e.g., elastically. In the normal case, the drive is locked by the locking unit only to be released by said locking unit when a crash occurs. Said crash event is sensed by a crash sensor. A current crash sensor provided anyway for enabling the air bags may be used for this purpose as well. Or a separate crash sensor may be provided. It is responsive to the threshold at which the air bags are usually also enabled.
The locking unit has to be capable of releasing the drive as quickly as possible so that the second structural component can be moved as fast as possible from the normal position to the forwardly moved position upon sensing a crash. The locking unit is responsive to electrical activation, it receives an electrical signal from the crash sensor and becomes actuated when this signal occurs.
On the headrest according to the document DE 102 15 054 A1 mentioned herein above, the locking unit comprises a magnetic clamp, meaning the locking unit is configured to have magnetic holding force. The magnetic holding force is preferably applied by a permanent magnet. There is provided a coil that generates an opposing magnetic field when the current flows through it in the event of a crash. This opposing field weakens the force of the permanent magnet to such an extent that the locking unit disengages and the second structural component is allowed to move with respect to the first structural component.
For practical operation however, such a magnetic clamp is not safe enough. It relies for operation on frictional adherence. If its holding force is reduced by an annoyance such as acceleration or what is called misuse or vandalism, and if the extent of this reduction corresponds to the reduction effected by the current-carrying coil, the headrest is enabled. This however is useless, untimely and incommodes the passengers.
Another disadvantage of the prior art magnetic clamp is that the cooperating surfaces have to very accurately match. Dirt also has to be prevented from soiling the adhering surfaces. If possible, any gap between said adhering surfaces should be avoided.